1. Field of the Invention
The present invention relates to a substrate for a magnetic recording medium and a magnetic recording medium using the substrate.
2. Discussion of Background
There has been a demand for a magnetic recording medium (hereinbelow, referred to as a disk) having a high recording density, along with which it has been necessary to reduce a flying height of a magnetic head (hereinbelow, referred to as a head). In recent years, a flying height of a head from a disk surface of 500 .ANG. or lower, desirably, 300 .ANG. or lower is required. Accordingly, it is necessary that the disk surface is extremely flat in a data zone where the recording/reading of data are effected.
On the other hand, in a contact-start-stop (CSS) method, a head takes off and lands on a disk surface when the rotation of the disk is started or stopped. The disk surface is not smooth, but there are minute projections and recesses called a texture formed to prevent the head from sticking on the disk surface at the time of taking off and landing.
In order to satisfy both requirements of preventing the head from sticking and of the head taking off with a low flying height while a high recording density is maintained, it is necessary to form the texture of minute projections and recesses wherein a dispersion in the height of the projections is small. However, it is not easy to form such texture.
Accordingly, there is proposed a disk wherein the disk surface is divided into a zone in which the head rests at the time of stopping the disk (i.e., a landing zone) and a zone for effecting recording/reading of data (i.e., a data zone), and the texture is formed only in the landing zone while the data zone is left smooth.
The landing zone is formed in an inner peripheral area or an outer peripheral area with a predetermined width on a surface of a doughnut-like disk. The area other than the landing zone in the disk surface is substantially occupied by the data zone.
To form the texture in the disk surface, the texture is formed in a surface of a substrate to be used for the disk. Although a magnetic layer and other required layers are formed on the substrate, the texture formed in the substrate should correctly be transferred on the disk surface. In a case of forming the landing zone in the disk surface, the texture can be formed in the substrate surface at the region just below the landing zone in the disk surface (it is referred to as a landing zone of substrate). In this case, it is unnecessary to form the texture in the data zone in the disk and the data zone has a smooth surface. Accordingly, the corresponding area in the substrate (it is referred to as a data zone of substrate) is also smooth, and such material is suitable for a substrate for a magnetic disk, which is made of a non-magnetic brittle material such as glass having a feature of smoothness in its surface. Accordingly, a substrate for a magnetic recording medium which is formed of such material and has the texture having minute projections and recesses wherein a dispersion in the height of the projections is small, is envisioned.
In forming the texture in the surface of the substrate, there have been proposed a mechanical method using an abrasive material on a substrate of aluminum alloy with NiP plating or a glass substrate, or a chemically treating method to a glass substrate or the like. However, these methods have a disadvantage that the height of projections is not uniform thereby preventing the lowering of a flying height of the head.
Besides the above-mentioned methods, U.S. Pat. No. 5,062,021, U.S. Pat. No. 5,108,781 and Japanese Unexamined Patent Publication JP-A-8-106630 propose a method of forming a texture by using a laser. Further, a paper "A New Laser Texturing Technique for High Performance Magnetic Disk Drives" IEEE Trans. Mag., Vol.31, pp2946-2951,1995 describes related art.
In these documents, metal or alloy is used as the material to be processed by laser. Further, in these documents, the mechanism of forming a texture by applying a laser to such material is as follows. On irradiating laser, there takes place a temperature distribution in an irradiated region with the result of a distribution of surface tension in the radiated area, whereby there occurs a re-arrangement of compositions in correspondence to the surface tension, and solidification follows. In these prior art documents, there is no consideration of a substrate constituted by a brittle material such as a glass substrate, a glass ceramics substrate or a carbon substrate.
EP 0652554A discloses a method of forming a texture by irradiating a laser beam on the surface of a brittle material having a thermal shock fluence threshold level. However, there is no special proposal to control the height of minute projections and the depth of minute recesses in order to form a uniform texture wherein a dispersion of the height of the projections is small.